It is well known that photographic films can suffer from deleterious effects owing to excessively close contact between adjacent layers of film or between a layer of film and an attached layer of a second flexible material, for example a paper strip backing in the case of roll film. These deleterious effects take the form of sticking, or blocking, of one layer to another or merely of a tendency to accumulate an electrostatic charge which is dissipated when the two layers are separated. In the latter case a visible discharge of light can arise which can fog the film.
It is therefore well known to incorporate in one or more layers and on one or more sides of the photographic film, an antiblocking agent in the form of finely divided hard particles, either of a mineral or more usually of a polymeric nature. Most typically these particles are spherical and often their particle size is closely controlled, since it is well known that very large particles (e.g., having a diameter of about 10 micrometers or more) may be readily seen in photographic prints prepared from the photographic film in which the particles are included. However, a particle size of about 2 to about 8 microns is necessary in order to achieve a useful separation effect between two adjacent film layers, which requires that typically between about 1 and about 7 microns of the particle diameter protrudes from the film.
Owing to the relatively large size of the above-described particles, it is quite commonly found that when such a film is printed, an effect known as "Starry Night Effect" is observed. It consists of a series of visible images of the antiblock particles themselves. This effect may be particularly noticeable when the refractive index of the antiblock particle is very different from that of the photographic coating layer in which the particle is embedded.